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Roadmap
*car c = malloc(sizeof(car)); c->miles = 100; c->gals = 17; float mpg = get_mpg(c); free(c); Car c = new Car(); c.setMiles(100); c.setGals(17); float mpg = c.getMPG(); get_mpg: pushq %rbp movq %rsp, %rbp ... popq %rbp ret
C: Java:
Assembly
language:
Machine
code:
0111010000011000 100011010000010000000010 1000100111000010 110000011111101000011111
Computer
system:
OS:
Memory & data Integers & floats Machine code & C x86 assembly Procedures & stacks Arrays & structs Memory & caches Processes Virtual memory Memory allocaJon Java vs. C
How does execuJon Jme grow with SIZE?
int array[SIZE]; int A = 0; for (int i = 0 ; i < 200000 ; ++ i) { for (int j = 0 ; j < SIZE ; ++ j) { A += array[j]; } }
SIZE
TIME
Plot
Making memory accesses fast!
¢ Cache basics ¢ Principle of locality ¢ Memory hierarchies ¢ Cache organizaJon ¢ Program opJmizaJons that consider caches
Problem: Processor-‐Memory Bo[leneck
Main
Memory
CPU Reg Processor performance doubled about every 18 months Bus bandwidth evolved much slower Core 2 Duo: Can process at least 256 Bytes/cycle Core 2 Duo: Bandwidth 2 Bytes/cycle Latency 100 cycles Problem: lots of wai4ng on memory 5
cycle = single fixed-‐Jme
Spring 2014 machine step Memory and Caches
Cache
¢ English definiJon: a hidden storage space for provisions, weapons, and/or treasures ¢ CSE definiJon: computer memory with short access Jme used for the storage of frequently or recently used instrucJons or data (i-‐cache and d-‐cache) more generally, used to opJmize data transfers between system elements with different characterisJcs (network interface cache, I/O cache, etc.)
General Cache Mechanics
Cache^8 9 14 Memory Larger, slower, cheaper memory viewed as parJJoned into “blocks” or “lines” Data is copied in block-‐sized transfer units Smaller, faster, more expensive memory caches a subset of the blocks (a.k.a. lines)
General Cache Concepts: Miss
Cache^8 9 14 Memory
Request: 12 Data in block b is needed
Block b is not in cache:
Miss!
Block b is fetched from
memory
Request: 12 12
Block b is stored in cache
- Placement policy: determines where b goes
- Replacement policy: determines which block gets evicted (vicAm)
Why Caches Work
¢ Locality: Programs tend to use data and instrucJons with addresses near or equal to those they have used recently
Why Caches Work
¢ Locality: Programs tend to use data and instrucJons with addresses near or equal to those they have used recently ¢ Temporal locality: § Recently referenced items are likely
to be referenced again in the near future
¢ SpaJal locality? block
Why Caches Work
¢ Locality: Programs tend to use data and instrucJons with addresses near or equal to those they have used recently ¢ Temporal locality: § Recently referenced items are likely
to be referenced again in the near future
¢ SpaJal locality: § Items with nearby addresses tend
to be referenced close together in Ame
§ How do caches take advantage of this? block block
Locality Example
int sum_array_rows(int a[M][N]) { int i, j, sum = 0; for (i = 0; i < M; i++) for (j = 0; j < N; j++) sum += a[i][j]; return sum; } a[0][0] a[0][1] a[0][2] a[0][3] a[1][0] a[1][1] a[1][2] a[1][3] a[2][0] a[2][1] a[2][2] a[2][3]
Locality Example
int sum_array_rows(int a[M][N]) { int i, j, sum = 0; for (i = 0; i < M; i++) for (j = 0; j < N; j++) sum += a[i][j]; return sum; } a[0][0] a[0][1] a[0][2] a[0][3] a[1][0] a[1][1] a[1][2] a[1][3] a[2][0] a[2][1] a[2][2] a[2][3] 1: a[0][0] 2: a[0][1] 3: a[0][2] 4: a[0][3] 5: a[1][0] 6: a[1][1] 7: a[1][2] 8: a[1][3] 9: a[2][0] 10: a[2][1] 11: a[2][2] 12: a[2][3] stride-‐
Locality Example
int sum_array_cols(int a[M][N]) { int i, j, sum = 0; for (j = 0; j < N; j++) for (i = 0; i < M; i++) sum += a[i][j]; return sum; } a[0][0] a[0][1] a[0][2] a[0][3] a[1][0] a[1][1] a[1][2] a[1][3] a[2][0] a[2][1] a[2][2] a[2][3] 1: a[0][0] 2: a[1][0] 3: a[2][0] 4: a[0][1] 5: a[1][1] 6: a[2][1] 7: a[0][2] 8: a[1][2] 9: a[2][2] 10: a[0][3] 11: a[1][3] 12: a[2][3] stride-‐N
Locality Example
int sum_array_3d(int a[M][N][N]) { int i, j, k, sum = 0; for (i = 0; i < N; i++) for (j = 0; j < N; j++) for (k = 0; k < M; k++) sum += a[k][i][j]; return sum; } ¢ What is wrong with this code? ¢ How can it be fixed?
